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Mark 1, Nos. 1 and 2, were 31 calibers and two of the first steel tube guns that were built entirely in the United States. They were trunnioned guns, no liners and that fired bag ammunition. After the Spanish–American War was over they were modified to Mod 1 in 1901. A liner was inserted in the breech end and the trunnions were cut off. The Mod 1 consisted of tube, jacket, and 9 hoops. After these changes the gun was able to use the same ammunition as the later Mark 2 5-inch gun.[1][2]